Progesteronereceptorsof humantumorsof breast and gynecologicaloriginwere assayed with [6,7-3H]-17,21-di methyb-19-norpregna-4,9-diene-3,20-dione or [1,2,6,7-3H]progesterone plus a 100-fold excess of cortisol by the charcoal extraction method. Whereas both ligands gave concordantlevels of bindingfor the high-capacitysam pies,highernonspecific bindingof I 7,21-dimethyl-1 9-nor pregna-4,9-diene-3,20-dione to serumalbuminresultedin less precise estimates of low-binding-capacitysamples. Ammoniumsulfate precipitationof receptor and gra dient ultracentrifugationanalysis demonstratedthat the nonspecificbindingof 17,21-dimethyl-19-norpregna-4,9diene-3,20-dionewas nonprcipitabIe by 30% (NHj@SO4 andmigratedwithserumalbumin.That 17,21-dimethyl-19norpregna-4,9-diene-3,20-dione bindsto humanserumal buminwas further shownby gradient ultracentrlfugation analysisandthe charcoalextractionmethod. The importanceof addingan excessof cortisolto block binding to corticosteroid-bindingglobulin with [1,2,6,73H]progesterone to measure progesterone receptor bind ingwas redemonstrated. We concludethat presentlythe mostaccuratemethods described to measure progesterone receptors in crude cytosol are those that use labeled progesterone as ligand, provided that the nonspecific binding to corticosteroid binding globulin is blocked by an excess of unlabeled cortisol.